pH dependence of PDMS–PMAA IPN morphology and transport properties

The objectives of this study are to probe the pH responsiveness of polydimethyl siloxane (PDMS)/polymethacrylic acid (PMAA) interpenetrating polymer networks (IPNs), and to relate pH-dependent changes in permeation to morphological changes.

PDMS/PMAA IPNs of 70:30 mass ratio on a dry basis were prepared using a monomer immersion method Macromolecules 33 (10) (2000) 3714]. IPNs made by this method have previously been shown to exhibit a bicontinuous morphology at pH 7 with PMAA structural features of 10–200 nm in dimension Macromolecules 36 (6) (2003) 1962]. Equilibrium hydration, morphology and diffusion characteristics of the IPNs were investigated at different pH. Equilibrium hydration was gravimetrically determined to be in the range of 0.77–0.9 at pH 7 and 0.42–0.5 at pH 3, corresponding to average hydrations of 0.93 and 0.74 of the PMAA component, respectively. Morphology imaged using laser scanning confocal microscopy (LSCM) showed that the area fraction of fluorescein isothiocyanate labelled dextran (FITC-dextran) accessible regions decreased from pH 7 to 3, consistent with hydration data. Additionally at pH 3, the hydrogel domain appeared to be disconnected. Vitamin B₁₂ (VB₁₂) permeability through the IPN was found to be 1.7 × 10⁻⁷ cm²/s at pH 7, and below detection (<10⁻¹³ cm²/s) at pH 3, consistent with the observed morphology. Dynamic responses in hydration and permeation to pH changes suggests a surface driven phenomenon with response times that are expected to be thickness-dependent.